Editorial: replace UTF-8 encode with isomorphic encode

This more clearly indicates the input is (supposed to be) ASCII safe.

fetch.bs

@@ -3598,10 +3598,8 @@ Range Requests</cite>. [[HTTP-RANGE]] However, this is not widely supported by b
 
    <li><p>If <var>httpRequest</var>'s <a for=request>referrer</a> is a <a for=/>URL</a>, then
    <a for="header list">append</a> `<code>Referer</code>`/<var>httpRequest</var>'s
-   <a for=request>referrer</a>, <a lt="url serializer">serialized</a> and <a>UTF-8 encoded</a>, to
-   <var>httpRequest</var>'s <a for=request>header list</a>.
-   <!-- XXX ideally we have an easier way to convert something ASCII-safe into bytes
-            concept-as-bytes -->
+   <a for=request>referrer</a>, <a lt="url serializer">serialized</a> and <a>isomorphic encoded</a>,
+   to <var>httpRequest</var>'s <a for=request>header list</a>.
 
    <li><p>If the <i>CORS flag</i> is set, <var>httpRequest</var>'s <a for=request>method</a> is
    neither `<code>GET</code>` nor `<code>HEAD</code>`, or <var>httpRequest</var>'s
@@ -5880,7 +5878,7 @@ method, when invoked, must run these steps:
  <a for=response>status</a> to <var>status</var>.
 
  <li><p><a for="header list">Set</a> `<code>Location</code>` to <var>parsedURL</var>,
- <a lt="url serializer">serialized</a> and <a>UTF-8 encoded</a>, in <var>r</var>'s
+ <a lt="url serializer">serialized</a> and <a>isomorphic encoded</a>, in <var>r</var>'s
  <a for=Response>response</a>'s <a for=response>header list</a>.
 
  <li><p>Return <var>r</var>.